This invention relates to a method for infusibilization of pitch fibers by heat treating, in an oxidizing atmosphere, pitch fibers obtained by melt spinning a spinning pitch prepared from coal or petroleum pitch.
In the case of producing carbon fibers by melt spinning a coal or petroleum pitch, pitch fibers prepared by spinning a pitch are heat meltable and, if the pitch fibers are carbonized as they are, they are molten and cannot retain the individual fibrous shape. Therefore, infusibilization treatment of pitch fibers to insolubilize and infusibilize the pitch fibers by introduction of oxygen into molecular structure of pitch in the pitch fibers to produce crosslinkage therein is conducted under controlled conditions before carbonization treatment.
For infusibilization of pitch fibers, there have been proposed a method of dipping pitch fibers in an aqueous solution of an oxidizing agent (Japanese Patent Kokoku (Post Exam Publn) Nos. 47-21904 and 47-21905) and a method of extracting a component of low softening point in pitch fibers with a solvent (Japanese Patent Kokoku (Post Exam Publn) No. 52-38855 and Japanese Patent Kokai (Laid Open Publn) No. 61-2824). Normally, however, there has been employed a method of infusibilization by heating pitch fibers in an oxidizing gas (such as air, oxygen, ozone, nitrogen dioxide or mixed gas thereof) to oxidize the fibers.
For heating pitch fibers in an oxidizing gas, the following methods have been proposed, namely, a method of putting pitch fibers in a container and infusibilizing the fibers in batchwise manner (Japanese Patent Kokai (Laid Open Publn) Nos. 60-151316 and 61-12917), a method of continuously introducing pitch fibers accumulated on a conveyor into an infusibilizing furnace and continuously infusibilize the fibers therein (Japanese Patent Kokai (Laid Open Publn) Nos. 51-60774, 55-90621 and 59-192723), a method of continuous infusibilization by putting pitch fibers in a container or suspending them in a container and introducing continuously or intermittently this container into an infusibilizing furnace (Japanese Patent Kokai (Laid Open Publn) Nos. 55-6547, 58-60019 and 60-126323).
According to the batchtype infusibilizing method, temperature of the infusibilizing furnace can be accurately controlled depending on the rising rate of softening temperature of pitch fibers with progress of infusibilization and generation of heat from pitch fibers with progress of infusibilization and thus infusibilization can be performed without damaging the characteristics of fibers and causing runaway reaction of pitch fibers. However, it is well known that heat treatment of batch process system is industrially disadvantageous and is not suitable for mass-production of carbon fibers. Infusibilization of pitch fibers by accumulating pitch fibers on a conveyor or in a container and continuously passing the pitch fibers through an infusibilizing furnace can be expected to accomplish higher productivity than the batchtype infusibilization. However, control of distribution of temperature in the furnace is difficult and so it is difficult to perform heat treatment of pitch fibers at proper heating rate. That is, in the case of a furnace of conveyor type, normally inside of the furnace is divided into a plurality of sections, in each of which are provided a ventilating apparatus and a temperature controlling apparatus and heat treatment is carried out by blowing an oxidizing gas adjusted to a given temperature upon pitch fibers or flowing the oxidizing gas through the pitch fibers. According to such furnace, naturally, the middle part of each section in lengthwise derection has relatively uniform and flat distribution of temperature while abrupt change of temperature and thus non-uniform distribution of temperature occur at boundary part of each section. Therefore, when conveyor or container is moved at a constant speed through the infusibilizing furnace, the pitch fibers on the conveyor or in the container undergo extremely non-uniform and stepwise change of temperature (heating rate). That is, heating rate is slow and thus infusibilization reaction is gentle in the middle part of each section. On the other hand, elevation of temperature is rapid and infusibilization reaction is abrupt in the boundary part of each section. Rapid elevation of infusibilization temperature causes the temperature of atmosphere to be close to softening point of pitch fibers and causes fusion bonding of fibers. Thus, not only characteristics of fibers are damaged, but also uncontrollable exothermic reaction is brought about depending on state of accumulation of fibers and state of ventilation. Therefore, operating conditions of infusibilizing furnace of such type (preset temperature in each section, speed of conveyor, ventilation condition and accumulation amount of pitch fibers) are limited on the basis of heating rate in boundary part between each section and thus, there must be employed such conditions which result in reduction of productivity, for example, reduction in conveyor speed and decrease in accumulated amount of pitch fibers.
The method of infusibilization by intermittently introducing containers in which pitch fibers are accumulated, into infusibilizing furnace or intermittently moving the containers through the furnace also suffers from the problem of temperature control.
That is, in the method of intermittently moving the container, pitch fibers undergo more stepwise change of heat treating temperature than in the continuous moving method, namely, they are repeatedly subjected to retention of a certain temperature and abrupt elevation of temperature. According to such heating method, infusibilizing reaction comprises combination of rapid and slow progresses and operating conditions of infusibilizing furnace are limited on the basis of rapid heating rate just after beginning of movement as mentioned for the method of continuous introduction of conveyor or container into infusibilizing furnace. For this reason, measures such as increase of residence time in each section and decrease of the amount of accumulation of pitch fibers on conveyors or in containers must be taken, which inevitably reduce productivity.
As explained above, the conventional infusibilizing methods all have defects. Especially, it is a essential problem in production of pitch-based carbon fibers that elevation of heat treating temperature of pitch fibers is not uniform and so the productivity is restricted in the industrially important method of continuous or semicontinuous infusibilization by continuously or intermittently introducing conveyors or containers into an infusibilizing furnace.